This
Exercise is used to strengthen the obliques. The obliques
are responsible for flexion, extension and rotaton of the
spine. Proper execution of this exercise requires a slow controlled
movement with full extension and a tight contraction. The
rectus abdominus and transverse play a big supporting role
in this movement as well. Make sure to use a ball that allows
your knees to be at a 90 angle or less. Using a ball on the
smaller side is safer than using one that is to big.

Starting
position: Lie on the ball with your feet pressed into
the ground and knees bent at about 90°.